A number of international standards define how information representing aural and visual stimuli can be encoded and formatted for recording and transmission, and how the encoded information can be received and decoded for playback. For ease of discussion, information representing aural and visual stimuli is referred to herein as audio and video information, respectively.
Many applications that conform to these standards transmit the encoded audio and video information as binary data in a serial manner. As a result, the encoded data is often referred to as a bitstream but other arrangements of the data are permissible. For ease of discussion, the term “bitstream” is used herein to refer to encoded data regardless of the data format or the recording or transmission technique that is used.
Two examples of these standards that are published by the International Standards Organization (ISO) are ISO/IEC 13818-7, Advanced Audio Coding (AAC), also known as MPEG-2 AAC, and ISO/IEC 14496-3, subpart 4, also known as MPEG-4 audio. These two standards share technical features that make them similar to one another for purposes of this disclosure.
Standards such as the MPEG-2 AAC and MPEG-4 audio standards define bitstreams that are capable of conveying encoded data representing one or more audio channels. The concept of an audio channel is well known. The conventional stereophonic playback system with two loudspeakers is a well-known example of a playback system capable of reproducing two audio channels, often referred to as the left (L) and right (R) channels. Multichannel playback systems for so-called home theatre applications are capable of reproducing additional channels such as the center (C), back-left-surround (BL), back-right-surround (BR) and low-frequency-effects (LFE) channels.
A system that is capable of playing back audio from an encoded bitstream must include a device that is capable of extracting encoded data from the bitstream and decoding the extracted data into signals representing the individual audio channels. The cost of hardware resources for memory and processing required to decode data and apply a synthesis filter to obtain an output signal is a significant portion of the total manufacturing cost of the decoding device. As a result, the power requirements and purchase price of a decoder is affected significantly by the number of channels the decoder is capable of decoding. In an effort to reduce power requirements and purchase price, audio system manufacturers build decoders that are capable of decoding only a desired subset of all channels that are defined in a bitstream standard. Referring to the MPEG-2 AAC and MPEG-4 audio standards as examples, bitstreams can convey encoded data representing from one to forty-eight audio channels but most if not all practical decoders can decode only a small fraction of the maximum number of channels.
A typical decoder will process a particular bitstream only if it has the capability to decode all of the encoded channels that are conveyed in that bitstream. If a typical decoder receives a bitstream that conveys data representing more audio channels than it can decode, that decoder essentially discards the encoded data in the bitstream and does not decode any of the channels. This unfortunate situation exists because the decoder does not have the logic necessary to select and process a subset of the channels conveyed by the bitstream in an intelligent manner.